dull as dishwater vs sing-song

dull as dishwater

adj
  • Boring; ordinary; exceedingly dull. 

sing-song

adj
  • Like a piece of sing-song; simple and melodic, song-like. 

noun
  • A kind of verse with a simple, song-like rhythm. 

  • An informal gathering involving group singing. 

How often have the words dull as dishwater and sing-song occurred in a corpus of books? (source: Google Ngram Viewer )